1. Technical Field
The present invention relates to a focus control apparatus and method.
2. Related Art
Many imaging apparatus of related art, such as a monitoring camera and a DVD (digital versatile disc) camera, have a built-in autofocus function that allows automatic focus adjustment. An example of a focusing method in an automatic focus function of this type is a contrast-based focus adjustment method using a fact that the amplitude of a contrast signal provided from captured video images is maximized when the images are brought into focus.
In an imaging apparatus, when a focusing lens is moved along the optical axis thereof, captured video images are defocused or brought into focus and the amplitude of the contrast signal changes accordingly. In a most fundamental contrast-based method, the focusing lens is temporarily moved along the optical axis thereof, and a direction in which images are brought into focus is detected based on the magnitude of the amplitude of the contrast signal before and after the movement, followed by movement of the focusing lens in the detected direction.
In focusing methods used in the autofocus function, it is difficult to bring an image of a point light source in a dim environment into focus in many cases. The reason for this is that an image of a point light source that is not brought into focus spreads as compared with an image of the point light source that is brought into focus and the lens position where the amplitude of the contrast signal is maximized is shifted from the position where the image is brought into focus in some cases. To solve the problem described above, JP-A-2007-065048 describes a technology for dividing a captured image into a plurality of areas, evaluating the luminance level in each of the areas, and selecting an exposure condition under which autofocus control is performed in accordance with the evaluated luminance level.
However, when only the exposure condition is changed, an image of a point light source may still spread or otherwise deforms depending on the lens position. It is therefore difficult to bring the image into focus in many cases.
The invention has been made in consideration of the point described above and proposes a focus control apparatus and method capable of achieving a focused state even when a subject is a point light source or an object under a similar condition.